A fuel cell is a device that directly converts the chemical energy of reactants (a fuel and an oxidant) into low-voltage d.c. electricity. Many of the operational characteristics of fuel cell systems are superior to those of conventional power generation. For a large number of fuel cell related applications, a high power density would be required. Because of the size limitation, the power density is especially critical to fuel cell applications involving portable or wireless electronics devices, such as consumer electronics, micro sensors, and micro electromechanical or micro fluidic systems. Another important application requiring a high power density is the fuel cell power system for a micro aero vehicle. In this case, the total power requirement is substantial and the size limitation is very stringent due to the small size of the vehicle. In recent years, direct liquid-feed fuel cells such as direct methanol fuel cells and direct formic acid fuel cells have been under intensive development primarily for portable applications. One of the most serious limitations related to a direct liquid-feed fuel cell is its very low power density per unit reactive surface area. Even with a heavy catalyst loading, the recent power density of a direct liquid-feed fuel cell is still on the order of 10 mW per square centimeter of reactive surface area. The power density per unit reactive surface area of a bioelectrocatalytic fuel cell is even much lower using a bio-fluid as the fuel. The power output per unit volume (power density) of a fuel cell can be expressed by the following relation:P=Apwhere P is the power output per unit volume (power density), p is the power output per unit reactive surface area, and A is the total reactive surface area per unit volume or specific reactive surface area. From the equation above, the power density of a fuel cell system can be substantially increased by significantly increasing the total reactive surface area A even if the power output per unit reaction surface area p is quite low. This is similar to the case in human body organ systems such as the cardiovascular circulatory system and respiratory lung system, which require a rapid exchange/reaction rate within a limited volume. To fulfill the goals of these organ systems, larger vessels that carry an exchange fluid would branch out into many small vessels/capillaries to increase the exchange/reaction surface area with the surrounding cells. Since these small vessels/capillaries would have an extremely thin wall, to enhance their mechanical strength and sustain a pressure differential across the capillary walls, they generally take a circular shape, in terms of a circular tube or spherical ball. As such, two of the key mechanical characteristics of a bio system requiring a high reaction rate per unit volume are: (1) micro vessels and (2) a circular cross section.
Even for a hydrogen fuel cell power system for automotive or aerospace propulsive applications, where the size requirement is less stringent and the power density currently achievable is considered to be reasonably high, the above discussed bio-related concept is still useful. It is well known that for a hydrogen/air fuel cell using a moderate amount of catalyst, the cell voltage drops sharply as the current density is increased. This indicates that a higher current density causes a larger irreversibility in the fuel cell and significantly reduces the energy utilization rate of the fuel cell. For a fuel cell to work at a higher energy efficiency, it may need to run at a lower current density. However, this low current density would substantially reduce the power density of the fuel cell stack and render the stack to be impractical for transportation applications. With above-discussed concept, a fuel cell stack can be constructed such that it would have a significantly increased reaction surface area per unit volume and the power density of the fuel cell stack is substantially improved. The end result is that the fuel cell stack can operate at a lower current density with a much improved energy efficiency without materially increasing the size/weight of the stack.